1. Field of the Invention
The present invention relates to a method for producing an electrolytic solution suitable for a redox battery, in particular, high purity trivalent vanadium, tetravalent vanadium and/or mixture of trivalent and tetravalent vanadium electrolytic solution.
2. Description of the Related Art
In recent years, global environmental pollution such as acid rain, the destruction of the ozone layer by fluorocarbons, and the greenhouse effect due to an increase in carbon dioxide in the atmosphere are being focussed upon as a problems for all mankind. In the midst of this state of affairs, the movement to make the fullest possible use of solar energy, an inexhaustible and clean form of energy that is friendly towards the earth, has increased considerably. Examples of this include solar batteries, power generation making use of solar heat or heat reclamation, wind-turbine generation, and wave power generation (power generation making use of the energy of ocean currents, or temperature differences of seawater).
Of all of these, it is solar batteries, with a remarkable revolution in technology, that show signs of heading towards the time when they are ready for genuine practical applications, through improvements in their efficiency and a significant lowering in price. Currently, use of solar batteries is restricted to rather small-scale applications such as in powering road signs and communications relays, but rapid developments are also expected through envisioned solar energy cities and the implementation of designs to lay fields of batteries in the oceans or deserts. However, the power output of all of these power generation methods making use of solar energy is affected by climatic conditions, thus making stable and trustworthy production of electrical power impossible. Coordinated use of solar energy and reliable, effective batteries are required, and the realization thereof has been long awaited.
Moreover, electrical power may be easily converted into other types of energy, is easy to control, and causes no environmental pollution at the time of its consumption, and it is for these reasons that the percentage of total consumption taken up by electrical power is increasing every year. The distinguishing characteristic of electrical power is that its production and consumption are simultaneous with each other, and that it cannot be stored. It is for this reason that, at present, highly efficient nuclear power generation and advanced thermal power generation are being operated at the highest possible efficiency ratings, and that the large increase in demand for electricity during daylight hours is being met by small-scale thermal and hydropower generation suitable for generating power in response to fluctuations in consumption of electrical power. Thus, the current state of affairs is such that excess energy is being produced at night. The power generation world is earnestly hoping for development of technology that will make it possible to store this excess energy at night and use it efficiently during the day.
From circumstances such as those above, all types of secondary batteries have been studied as a method of storing electrical energy which does not pollute the environment and as an energy with a wide variety of applications. Redox batteries have received special attention as a high-volume stationary battery capable of operating at room temperature and atmospheric pressure. Redox batteries pass electrically active materials of positive and negative solution to the cells with flow-through electrodes, and are charged and discharged through a redox reaction. They thus have a comparatively longer life than normal secondary batteries, with minimized self-discharging, possess the advantages of being high in both reliability and safety and in having freely changeable electric capacity due to the separation of battery cells performing oxidation-reduction reaction and reservoirs storing electricity. Vanadium redox flow batteries using vanadium as both positive and negative electrodes have especially high output and readily recover even though there is a mixing of positive and negative electrolytic solutions through ion exchange membrane. Accordingly, the area of putting such batteries into the practical use has received the most attention in recent years.
A vanadium redox flow battery with tetravalent and pentavalent vanadium ion pairs as the positive electrode and trivalent and divalent vanadium ion pairs as the negative electrode was proposed by Professor M. Skylass-Kazacos of New South Wales University in Australia (Japanese Patent Laid-Open No. 62-186473, E. SUM "Journal of Power Sources", 15(1985), 179-190 and 16(1985), 85-95). This battery has a high output voltage of 1.4 V to 1.5 V, and is characterized by its high efficiency and high energy density, but as expensive vanadium must be used this has been viewed as being poorly suited for practical use.
In order to solve the above problems, the present inventors have proposed in Japanese Patent Laid-Open Nos. 4-149965 and 5-303973 a method for producing at low cost a vanadium electrolytic solution by reducing, in the presence of an inorganic acid, the vanadium compound recovered from the ash produced by combustion of heavy oil fuels, and a method in which a trivalent vanadium electrolytic solution and a tetravalent vanadium electrolytic solution are simultaneously prepared by reduction with sulfur in a sulfuric acid solution.
A redox battery comprises end plates; positive and negative carbon cloth electrodes provided between the end plates; and a separating membrane which is made of an ion exchange membrane and placed between the positive and negative carbon cloth electrodes. The tetravalent vanadium electrolytic solution for the positive electrode and the trivalent vanadium electrolytic solution for the negative electrode are supplied to the respective electrodes from respective tanks. In charging the redox battery, tetravalent vanadiums are gradually oxidized to become pentavalent vanadiums at the positive electrode and trivalent vanadiums are gradually reduced to become divalent vanadiums at the negative electrode. Discharge will be started at the time that tetravalent and trivalent vanadiums change to pentavalent and divalent vanadiums at the positive and negative electrodes, respectively. The electrolytic solutions for the positive and negative electrodes may be mixtures equivalently containing the tetravalent vanadium and trivalent vanadium; for example, tetravalent vanadium:trivalent vanadium may be 1:1 in both electrolytic solutions for the positive and negative electrodes; or tetravalent vanadium:trivalent vanadium may be 2:1 and 1:2 in the electrolytic solutions for the positive electrode and negative electrode, respectively. Therefore, there is a need for a method in which an electrolytic solution containing trivalent vanadiums is more economically produced than with electrolytic reduction.
The deterioration of the ion exchange membrane is one of the important subjects to solve in the industrialization of vanadium redox flow batteries. The present inventors have proposed, in Japanese Patent Laid-Open No. 4-4043, polysulfone and fluorocarbon resin ion exchange membranes having oxidation durability against pentavalent vanadium electrolytic solution and low membrane resistance, and enable high current density vanadium battery production. However, it has been revealed that the durability of the ion exchange membrane depends on not only the material itself but also on impurities in the electrolytic solution, especially alkaline earth metals and silicon compounds.
On the other hand, raw materials for high purity vanadium is expensive and it is difficult to obtain the required quantity. Therefore, the industrialization of vanadium redox flow batteries essentially requires a method for producing vanadium electrolytic solution by continuous purification and reduction of low purity vanadium raw materials that are inexpensive and abundant.
Purification of vanadium compounds is generally carried out by converting vanadium compounds to ammonium metavanadate, dissolving this into hot water, filtrating, and then crystallizing. However, in order to obtain high purity ammonium metavanadate, the crystallizing process must be repeated many times, which is industrially impractical. Another purification process, in which vanadium oxychloride is produced from raw vanadium compounds and distilled, is also not economical due to the complicated process involved. In particular, in the case of using low purity vanadium compounds recovered from the ash produced by combustion of heavy oil fuels as raw materials for the electrolytic solution, it is difficult to remove Si, no method for producing a high purity vanadium electrolytic solution with effective purification and reduction has been proposed and therefore, the development of a method for producing high purity vanadium electrolytic solution is a subject of great urgency for the industrialization of vanadium redox flow batteries.